diff --git a/lib/chat_models/chat_google_ai.ex b/lib/chat_models/chat_google_ai.ex
index 355c5ee2..a093b0eb 100644
--- a/lib/chat_models/chat_google_ai.ex
+++ b/lib/chat_models/chat_google_ai.ex
@@ -187,8 +187,17 @@ defmodule LangChain.ChatModels.ChatGoogleAI do
         nil ->
           nil
 
-        %Message{role: :system, content: content} ->
+        %Message{role: :system, content: content} when is_binary(content) ->
           %{"parts" => [%{"text" => content}]}
+
+        %Message{role: :system, content: content} when is_list(content) ->
+          # Extract text from ContentPart structures
+          text_content =
+            content
+            |> Enum.filter(&match?(%ContentPart{type: :text}, &1))
+            |> Enum.map(& &1.content)
+            |> Enum.join(" ")
+          %{"parts" => [%{"text" => text_content}]}
       end
 
     messages_for_api =
diff --git a/lib/chat_models/chat_mistral_ai.ex b/lib/chat_models/chat_mistral_ai.ex
index 55e36300..fc2a11dd 100644
--- a/lib/chat_models/chat_mistral_ai.ex
+++ b/lib/chat_models/chat_mistral_ai.ex
@@ -179,23 +179,49 @@ defmodule LangChain.ChatModels.ChatMistralAI do
 
   def for_api(%_{} = model, %Message{role: :assistant, tool_calls: tool_calls} = msg)
       when is_list(tool_calls) do
+    content = case msg.content do
+      content when is_binary(content) -> content
+      content when is_list(content) -> ContentPart.parts_to_string(content)
+      nil -> nil
+    end
+
     %{
       "role" => :assistant,
-      "content" => msg.content
+      "content" => content
     }
     |> Utils.conditionally_add_to_map("tool_calls", Enum.map(tool_calls, &for_api(model, &1)))
   end
 
-  def for_api(%_{} = model, %Message{role: :user, content: content} = msg)
+  def for_api(%_{} = _model, %Message{role: :user, content: content} = msg)
       when is_list(content) do
     # A user message can hold an array of ContentParts
     %{
       "role" => msg.role,
-      "content" => Enum.map(content, &for_api(model, &1))
+      "content" => ContentPart.parts_to_string(content)
     }
     |> Utils.conditionally_add_to_map("name", msg.name)
   end
 
+  # Handle messages with ContentPart content for non-user roles
+  def for_api(%_{} = model, %Message{content: content} = msg) when is_list(content) do
+    role = get_message_role(model, msg.role)
+
+    %{
+      "role" => role,
+      "content" => ContentPart.parts_to_string(content)
+    }
+    |> Utils.conditionally_add_to_map("name", msg.name)
+    |> Utils.conditionally_add_to_map(
+      "tool_calls",
+      Enum.map(msg.tool_calls || [], &for_api(model, &1))
+    )
+  end
+
+  # Handle ContentPart structures
+  def for_api(%_{} = _model, %ContentPart{type: :text, content: content}) do
+    content
+  end
+
   # ToolResult => stand-alone message with "role: :tool"
   def for_api(%_{} = _model, %ToolResult{type: :function} = result) do
     %{
diff --git a/lib/chat_models/chat_ollama_ai.ex b/lib/chat_models/chat_ollama_ai.ex
index 8a1196d4..50cf2329 100644
--- a/lib/chat_models/chat_ollama_ai.ex
+++ b/lib/chat_models/chat_ollama_ai.ex
@@ -42,6 +42,7 @@ defmodule LangChain.ChatModels.ChatOllamaAI do
   alias LangChain.ChatModels.ChatModel
   alias LangChain.ChatModels.ChatOpenAI
   alias LangChain.Message
+  alias LangChain.Message.ContentPart
   alias LangChain.Message.ToolCall
   alias LangChain.Message.ToolResult
   alias LangChain.MessageDelta
@@ -242,9 +243,15 @@ defmodule LangChain.ChatModels.ChatOllamaAI do
 
   def for_api(%Message{role: :assistant, tool_calls: tool_calls} = msg)
       when is_list(tool_calls) do
+    content = case msg.content do
+      content when is_binary(content) -> content
+      content when is_list(content) -> ContentPart.parts_to_string(content)
+      nil -> nil
+    end
+
     %{
       "role" => :assistant,
-      "content" => msg.content
+      "content" => content
     }
     |> Utils.conditionally_add_to_map("tool_calls", Enum.map(tool_calls, &for_api(&1)))
   end
@@ -292,11 +299,25 @@ defmodule LangChain.ChatModels.ChatOllamaAI do
   def for_api(%Message{role: :user, content: content} = msg) when is_list(content) do
     %{
       "role" => msg.role,
-      "content" => Enum.map(content, &for_api(&1))
+      "content" => ContentPart.parts_to_string(content)
+    }
+    |> Utils.conditionally_add_to_map("name", msg.name)
+  end
+
+  # Handle messages with ContentPart content for non-user roles
+  def for_api(%Message{content: content} = msg) when is_list(content) do
+    %{
+      "role" => msg.role,
+      "content" => ContentPart.parts_to_string(content)
     }
     |> Utils.conditionally_add_to_map("name", msg.name)
   end
 
+  # Handle ContentPart structures
+  def for_api(%ContentPart{type: :text, content: content}) do
+    content
+  end
+
   defp get_tools_for_api(nil), do: []
 
   defp get_tools_for_api(tools) do
diff --git a/lib/chat_models/chat_open_ai.ex b/lib/chat_models/chat_open_ai.ex
index 6084ce38..66409403 100644
--- a/lib/chat_models/chat_open_ai.ex
+++ b/lib/chat_models/chat_open_ai.ex
@@ -536,6 +536,11 @@ defmodule LangChain.ChatModels.ChatOpenAI do
     raise LangChainError, "PromptTemplates must be converted to messages."
   end
 
+  # Handle ContentPart structures directly
+  def for_api(%_{} = model, %ContentPart{} = part) do
+    content_part_for_api(model, part)
+  end
+
   @doc """
   Convert a list of ContentParts to the expected map of data for the OpenAI API.
   """
diff --git a/lib/chat_models/chat_perplexity.ex b/lib/chat_models/chat_perplexity.ex
index 046c0428..9e965a87 100644
--- a/lib/chat_models/chat_perplexity.ex
+++ b/lib/chat_models/chat_perplexity.ex
@@ -45,6 +45,7 @@ defmodule LangChain.ChatModels.ChatPerplexity do
   alias LangChain.ChatModels.ChatModel
   alias LangChain.Message
   alias LangChain.MessageDelta
+  alias LangChain.Message.ContentPart
   alias LangChain.Message.ToolCall
   alias LangChain.TokenUsage
   alias LangChain.LangChainError
@@ -283,9 +284,15 @@ defmodule LangChain.ChatModels.ChatPerplexity do
   """
   @spec for_api(t(), Message.t()) :: %{String.t() => any()}
   def for_api(%ChatPerplexity{}, %Message{} = msg) do
+    content = case msg.content do
+      content when is_binary(content) -> content
+      content when is_list(content) -> ContentPart.parts_to_string(content)
+      nil -> nil
+    end
+
     %{
       "role" => msg.role,
-      "content" => msg.content
+      "content" => content
     }
   end
 
diff --git a/lib/chat_models/chat_vertex_ai.ex b/lib/chat_models/chat_vertex_ai.ex
index ab2a464c..069a0d0c 100644
--- a/lib/chat_models/chat_vertex_ai.ex
+++ b/lib/chat_models/chat_vertex_ai.ex
@@ -216,20 +216,25 @@ defmodule LangChain.ChatModels.ChatVertexAI do
   end
 
   defp for_api(%Message{role: :system} = message) do
-    %{"parts" => %{"text" => message.content}}
+    # System messages should return a single text part, not a list
+    case get_message_contents(message) do
+      [%{"text" => text}] -> %{"parts" => %{"text" => text}}
+      _ -> %{"parts" => %{"text" => message.content}}
+    end
   end
 
   defp for_api(%Message{role: :user, content: content}) when is_list(content) do
     %{
-      "role" => "user",
+      "role" => map_role(:user),
       "parts" => Enum.map(content, &for_api(&1))
     }
   end
 
   defp for_api(%Message{} = message) do
+    content_parts = get_message_contents(message) || []
     %{
       "role" => map_role(message.role),
-      "parts" => [%{"text" => message.content}]
+      "parts" => content_parts
     }
   end
 
@@ -257,9 +262,9 @@ defmodule LangChain.ChatModels.ChatVertexAI do
 
   defp for_api(%ContentPart{type: :file_url} = part) do
     %{
-      "file_data" => %{
+      "fileData" => %{
         "mimeType" => Keyword.fetch!(part.options, :media),
-        "file_uri" => part.content
+        "fileUri" => part.content
       }
     }
   end
diff --git a/test/chat_models/chat_mistral_ai_test.exs b/test/chat_models/chat_mistral_ai_test.exs
index f729a75e..ad9abefd 100644
--- a/test/chat_models/chat_mistral_ai_test.exs
+++ b/test/chat_models/chat_mistral_ai_test.exs
@@ -4,6 +4,7 @@ defmodule LangChain.ChatModels.ChatMistralAITest do
   alias LangChain.ChatModels.ChatMistralAI
   alias LangChain.Message
   alias LangChain.MessageDelta
+  alias LangChain.Message.ContentPart
   alias LangChain.Message.ToolCall
   alias LangChain.LangChainError
   alias LangChain.TokenUsage
@@ -113,7 +114,7 @@ defmodule LangChain.ChatModels.ChatMistralAITest do
 
       assert [%Message{} = msg] = ChatMistralAI.do_process_response(model, response)
       assert msg.role == :assistant
-      assert msg.content == "Hello User!"
+      assert msg.content == [ContentPart.text!("Hello User!")]
       assert msg.index == 0
       assert msg.status == :complete
     end
@@ -315,8 +316,8 @@ defmodule LangChain.ChatModels.ChatMistralAITest do
 
       result = ChatMistralAI.do_process_response(model, response)
 
-      assert [%Message{role: :assistant, content: "Hello from Mistral!", status: :complete}] =
-               result
+      assert [%Message{role: :assistant, status: :complete} = message] = result
+      assert message.content == [ContentPart.text!("Hello from Mistral!")]
     end
   end
 
diff --git a/test/chat_models/chat_ollama_ai_test.exs b/test/chat_models/chat_ollama_ai_test.exs
index 99ea0f96..a8e6a3e8 100644
--- a/test/chat_models/chat_ollama_ai_test.exs
+++ b/test/chat_models/chat_ollama_ai_test.exs
@@ -6,6 +6,7 @@ defmodule ChatModels.ChatOllamaAITest do
   alias LangChain.ChatModels.ChatOllamaAI
   alias LangChain.Function
   alias LangChain.FunctionParam
+  alias LangChain.Message.ContentPart
 
   use Mimic
 
@@ -600,7 +601,7 @@ defmodule ChatModels.ChatOllamaAITest do
 
       assert %Message{} = struct = ChatOllamaAI.do_process_response(model, response)
       assert struct.role == :assistant
-      assert struct.content == "Greetings!"
+      assert struct.content == [ContentPart.text!("Greetings!")]
       assert struct.index == nil
     end
 
diff --git a/test/chat_models/chat_perplexity_test.exs b/test/chat_models/chat_perplexity_test.exs
index c60afc51..4e3027c9 100644
--- a/test/chat_models/chat_perplexity_test.exs
+++ b/test/chat_models/chat_perplexity_test.exs
@@ -6,6 +6,7 @@ defmodule LangChain.ChatModels.ChatPerplexityTest do
   alias LangChain.ChatModels.ChatPerplexity
   alias LangChain.Message
   alias LangChain.MessageDelta
+  alias LangChain.Message.ContentPart
   alias LangChain.TokenUsage
   alias LangChain.LangChainError
   alias LangChain.Function
@@ -430,7 +431,7 @@ defmodule LangChain.ChatModels.ChatPerplexityTest do
 
       assert %Message{} = message = ChatPerplexity.do_process_response(model, response)
       assert message.role == :assistant
-      assert message.content == "Hello!"
+      assert message.content == [ContentPart.text!("Hello!")]
       assert message.index == 1
       assert message.status == :complete
     end
@@ -451,7 +452,7 @@ defmodule LangChain.ChatModels.ChatPerplexityTest do
 
       assert %Message{} = struct = ChatPerplexity.do_process_response(model, response)
       assert struct.role == :assistant
-      assert struct.content == "Some of the response that was abruptly"
+      assert struct.content == [ContentPart.text!("Some of the response that was abruptly")]
       assert struct.index == 0
       assert struct.status == :length
     end
diff --git a/test/chat_models/chat_vertex_ai_test.exs b/test/chat_models/chat_vertex_ai_test.exs
index fe0d689a..9dcad7d4 100644
--- a/test/chat_models/chat_vertex_ai_test.exs
+++ b/test/chat_models/chat_vertex_ai_test.exs
@@ -130,9 +130,9 @@ defmodule ChatModels.ChatVertexAITest do
                    "parts" => [
                      %{"text" => "User prompt"},
                      %{
-                       "file_data" => %{
-                         "file_uri" => "example.com/test.pdf",
-                         "mime_type" => "application/pdf"
+                       "fileData" => %{
+                         "fileUri" => "example.com/test.pdf",
+                         "mimeType" => "application/pdf"
                        }
                      }
                    ],